This proposal is focused on developing and characterizing new gene therapy treatments for Human Herpesvirus 8 (HHV-8) infections and Kaposi's sarcoma (KS) using a conditionally toxic gene, the herpesvirus thymidine kinase (HSV-TK). This gene will be transduced via retroviral vectors into a unique panel of human cell lines distributed by InCell: a human mesenchymal progenitor (MP) cell line permissive to HHV-8, a persistently infected MP cell line and a KS cell line positive for HHV- 8. The following aims are proposed: 1) Compare the toxicity of current antiviral nucleoside drugs in the parental and HSV-TK transduced cell lines. 2) Determine the magnitude and mechanism of the HSV-TK/drug- induced bystander effect. 3) Design HHV-8 and cytokine inducible retroviral vectors. The cytotoxic properties and metabolite levels of antiviral nucleoside drugs will be compared in parental and HSV-TK- transduced cell lines treated with ganciclovir (GCV). HSV-TK-transduced and non-transduced cell lines will be co-cultured in varying proportions and the GCV-mediated "bystander effect" studied. Lastly, unique retroviral constructs containing HHV-8 or cytokine inducible promoters will be constructed and tested. Commercially and clinically, the successful implementation of the proposed experiments will result in important preclinical efficacy data for novel treatment of KS and other cancers. PROPOSED COMMERCIAL APPLICATION New therapeutic and/or preventive biologicals for AIDS patients or other individuals with Kaposi's sarcoma should result from support of this project.